The present invention relates, in general, to programmable capacitor arrays and, more particularly, to a programmable capacitor array in which each capacitor of the array can be electrically isolated from other capacitors of the array.
Programmable capacitors are commonly used in such prior art applications as switched-capacitor filters, switched-capacitor gain stages, and switched-capacitor analog-to-digital (A/D) converters. In one type of switched-capacitor application, an operational amplifier is coupled to an active circuit node by means of a capacitor, wherein an output terminal of the operational amplifier is coupled to a first terminal of the capacitor while a second terminal of the capacitor is switched between the active circuit node and a ground potential. In yet another switched-capacitor application, the operational amplifier is coupled to the active circuit node by means of a parallel combination of capacitors. More particularly, the parallel combination of capacitors is coupled such that the first terminals of the capacitors are coupled together and the second terminals of the capacitors are selectively coupled to the active circuit node or to the ground potential. In other words, the capacitance between the output of the operational amplifier and the active circuit node may be selected by coupling the desired second terminals of the capacitors to the active circuit node. A drawback of this type of circuit configuration is that the operational amplifier drives each of the capacitors regardless of whether their second terminals are switched to the active circuit node or to the ground potential. In particular, the speed of the operational amplifier is decreased when a large number of capacitors are coupled to the output node of the operational amplifier. In addition, coupling a large number of capacitors to the output node of the operational amplifier may decrease the stability of the operational amplifier.
Accordingly, it would be advantageous to have a method and a means for decoupling individual capacitors from a parallel combination of capacitors and from the output terminal of an operational amplifier.